In many conventional mark reader systems, a single light source is employed to illuminate an entire document and the light reflected or transmitted from discrete locations on the document is monitored by a plurality of light detecting devices positioned relative to each of the document locations. The signal developed by each of the light detecting devices provides an indication of the presence or absence of a mark at the corresponding document location. The accuracy of such a system is dependent upon the electronics associated with each of the light detecting devices, and the uniform document illumination from the light source.